function Person() {
  this.think = function (callback) {
    setTimeout(function () {
      console.log("person is thinking");
      callback();
    }, 3000);
  };
  this.answer = function () {
    console.log("I am an answer");
  };
}
var person = new Person();
person.think(function () {
  console.log("think 3秒……");
});
person.answer();